The present invention relates to a method for producing a blank for a wide flange beam by use of a break-down mill of a shaped steel rolling line using a flat slab as a raw material element to be rolled.
In the production of a wide flange beam, a beam blank rolled by a blooming mill heretofore has been used as a raw material element to be rolled and has been reheated in a heating furnace and then rolled into the desired steel shaping in a shape steel plant. Recently, however, there has become widely employed a heat rolling operation in which a continuous-cast flat slab is used as the raw material element to be rolled and is heated in a heating furnace and then rolled into the desired wide flange beam in one operation step in the steel shaping plant.
In an ordinary steel shaping plant, a flat slab heated and soaked in a heating furnace to a suitable temperature of 1,200.degree. C. or above is rough-rolled by a break-down mill into a beam blank and thereafter rolled by a rougher universal mill having an edging mill in a latter stage and by a finishing universal mill into a desired wide flange beam.
The ordinary break-down mill has a pair of rolls forming first, second and third box calibers having sequentially larger widths and a sizing caliber of a rough H shape. The flat slab is fed to the first and the second box calibers with the widthwise direction thereof maintained vertically, whereby the flat slab is edged in the widthwise direction (about twice for each caliber) so as to be widened in the portions corresponding to flanges into a dog-bone-shaped workpiece. Thereafter, the dog-bone-shaped workpiece if fed alternately to the sizing caliber to be rolled into a beam blank after about 15 passes so as to be fed to universal mills in the succeeding steps.
The beam blank produced in this way has, however, the following problems:
(1) Continuous edging of a flat slab using a box caliber results in an increased central thickness at the top and the bottom thereof coupled with a fish-tail-like closing form in the upper and the lower edges since the slab is elongated longitudinally in the upper and the lower edges while it is not elongated in the central area at the top and the bottom. If such workpiece is rolled by the sizing caliber, a fin will occur on the sides of the resultant beam blank at the top and, particularly, at the bottom thereof, and such fin will remain in the final product as a defect.
(2) The continuous edging of the flat slab gradually decreases the rectangular ratio (the term rectangular ratio used herein and in the appended claims is defined as plate width/plate thickness or web height/web thickness). That is, since the plate thickness (or web thickness) is constant, the rectangular ratio decreases at each pass and the reduction effect reaches the central portion of the workpiece to thereby reduce the dog-bone effect. Accordingly, it is difficult to produce a wide flange beam having a flange width larger than the slab thickness.